I don't know if he is never (or rarely) the first option and Stewart doesn't ever get through his reads to where Blackmon is, or if Blackmon isn't getting open. While I have noticed him lining up out on the field fairly regularly, I admit that I haven't watched him run his routes to see whether or not he was open when he was in there. So I couldn't say whether or not he should be getting the ball more.
I will say that from what I have observed, Blackmon isn't as good of a WR as Leslie or Matthews are (at least right now). If he's injured, I hope he recovers. If he needs to learn the offense better, I hope that happens quickly. If there is something mental he needs to change, I hope he does it. I really like his potential (based on his physical abilities). I hope he can capitalize on it.